The present invention is an improvement over the invention described in commonly assigned U.S. Pat. No. 4,166,008, the disclosure of which is incorporated herein by reference.
U.S. Pat. No. 4,166,008 describes an improvement in the known system for recovery and purification of acrylonitrile and methacrylonitrile produced by the ammoxidation of propylene or isobutylene. In accordance with this improvement, a vaporous sidestream is removed from the lower fourth of the column whose bottoms product is used as the quench liquid (extractive distillation column or stripper) for quenching the ammoxidation reactor effluent. By withdrawing this sidestream, it was found that the concentration of heavy organics in the quench bottom streams could be significantly increased while at the same time the total mass flow of this stream significantly decreased. Since the quench tower bottoms are normally disposed of by incineration, this drastic decrease in the amount of quench tower bottoms represented a significant energy savings.
Unfortunately, a penalty associated with this approach is that additional heat must be supplied to the column from which the side stream is taken to account for the associated heat loss. For example, as taught in column 4 of U.S. Pat. No. 4,166,008, a 12% increase in heat input was necessary in the specific embodiment described.
Normally, heat is supplied to the extractive distillation column and stripper column by reboilers using low pressure steam as the heat source. With higher and higher energy costs, it is always desirable to reduce the usage of low pressure steam as much as possible so that excess low pressure steam can be used for other beneficial purposes.
Accordingly, it is an object of the present invention to provide a new system for accomplishing the same results achieved in U.S. Pat. No. 4,166,008, i.e. significant reduction in the amount of quench tower bottoms produced by a technique which requires significantly less energy than the technique described in that patent.